The past several years have seen remarkable improvement in laser technology. Much of this is due to improvements in material growth, device design and device fabrication. Despite significant improvements in efficiencies, it is becoming increasingly clear that III-nitride laser are limited by the ability to dissipate heat and thus must run at significantly reduced pulse length and duty cycle. Recent thermal simulations indicate that the substrate is not the primary source of the thermal impedance, rather it is the ability of the GaN to locally spread the heat to the surrounding material and substrate due to the extraordinarily high power dissipation density in the light generation device region.
Many groups have studied CVD polycrystalline diamond films. More recently nanocrystalline diamond (NCD) films have gained significant attention due to their high nucleation density and excellent thermal, mechanical, and electrical properties. See e.g., J. Philip, P. Hess, T. Feygelson, et al. “Elastic, Mechanical, and Thermal Properties of Nanocrystalline Diamond Films,” Journal of Applied Physics 93 pp. 2164-2171 (2003)
Much different from the CVD polycrystalline diamond films from the past, NCD films can be extremely smooth, dense and have very low in-grown stress. The improvement in NCD films is a direct result of a new seeding process based on the uniform coating with ‘detonation nanodiamond’ seeds on the target substrate surface. The nanodiamond particles are on the order of 3-5 nm and with appropriate ‘seeding’ conditions very high surface densities (>1×1012 cm−2) can be achieved. Such high seeding densities result in high nucleation density and very high quality, pin-hole-free and dense films.
Factors such as deposition temperature, chemistry, and growth conditions also play a large role in NCD film quality. Key NCD film properties are surface roughness, nucleation interface density, pinhole density, thermal conductivity, electrical resistivity, dielectric loss, and impact on the host substrate/films. Obtaining the highest quality NCD films while minimizing the impact on the target heterostructures is one of the major goals of this program. High quality NCD seeding and film growth are typically obtained under conditions that may not be compatible with those of the host substrate.
Key issues include the optimum NCD seeding and film deposition conditions in order to maintain high quality 2DEG in the target films is critical and will be the first area investigated under the proposed program. Understanding the fundamental electrical behavior of diamond on GaN will be the second task investigated under the program. Optimizing the overall III-nitride laser structure to include a heat spreading layer and a thin first dielectric layer. Diamond, like GaN, is a wide bandgap material and can have very high resistivity, however the effective band offset between diamond and GaN may be type II and thus tunneling into the diamond may be an issue. This impacts the dielectric between the GaN and the diamond, thus potentially reducing the heat spreading effect offered by diamond. Another consideration is that the preferred passivation layer for GaN is typically an ex-situ plasma deposited or in-situ grown SiN layer.
Thus, it is desirable to incorporate diamond films in III-nitride semiconductor lasers to improve thermal management in such devices.